Mutagen gives 20 levels to a wild tamed creature (5 each to health, stam, weight and melee) This means the tame (or a clone of the tame) must be something that has no parents in it's line, and no imprint bar on it's stats. The new stats can carry on to any offspring. You can only mutagen something once.
You *can* use mutagen on a bred tame, but it's a bit of a waste. It will only add 5 levels, 1 to each of the same above listed stats. These stats do NOT carry on to the offspring.